1. Field of the Invention
The present invention relates to an improvement of a susceptor which is employed to a vapor-phase growth apparatus to grow an epitaxial film onto the semiconductor wafer. More specifically, the present invention relates directly to a vapor-phase growth apparatus in which a through-hole portion extending to a rear side of the susceptor is provided at the most outer peripheral portion inside the wafer pocket in order to mount the wafer, and a raise in dopant concentration at the outer periphery of the grown epitaxial film can be controlled.
2. Description of the Prior Art
As to a vapor-phase growth apparatus in order to grow an epitaxial film onto the semiconductor wafer, there have been several conventional types of apparatus available; they may include (1) a vertical-type vapor-phase growth apparatus in which the susceptor being placed on a circular disc is heated from its bottom side, and (2) a single wafer type vapor-phase growth apparatus with which a good quality epitaxial film can be fabricated.
For example, inside the rectangular chamber being fabricated of quartz in said single wafer type vapor-phase growth apparatus, semiconductor wafer is mounted on the disc-shaped susceptor which is a graphite being coated with SiC. The semiconductor wafer is heated using a heater which is provided outside of the chamber in order to react with various types of reacting source gases passing through the chamber, resulting in growing the epitaxial film on the semiconductor wafer.
As seen in FIG. 3, said susceptor 5 is composed of a high purity graphite which is coated mainly by SiC. On the surface, a groove called as a wafer pocket 6 is formed in order to accomodate the silicon wafer. The size of said wafer pocket 6 is slightly larger than the dimension of the wafer 8 and the depth is about 1 mm. After placing the wafer 8 inside the wafer pocket 6, the susceptor is held in the reacting source gaseous flow at a predetermined temperature to generate the silicon epitaxial film layer 9 on the wafer surface.
Moreover, several improvements have been proposed in order to minimize the surface contact between the inner surface of the pocket and the wafer rear surface. These proposed improvements may include (1) a structure to contact-hold the wafer to a plurality of convex portions by forming a mesh-shaped shallow fine groove so-called a roulette, (2) making a tapered surface in order to confine the contact of the wafer at its outer periphery, or (3) using much coarser surface roughness of coated SiC surface than the surface roughness of the wafer.
As to a reacting source gas, a dopant source gas such as diborane (P type) or phosphine (N type) is added to a chloro-silane gas which is hydrogen diluted. Hence the silicon epitaxy as well as a bi-product of HCl are produced on the wafer surface through a heat CVD (chemical vapor deposition) reaction. As a result, although the silicon epitaxial growth on the wafer surface can proceed, the rear surface of said wafer is also exposed to diffusion reaction gas to create a Si—H—Cl atmosphere, which might furthermore lead to a precipitation/etching reaction in a microscopic scale.
For instance when the epitaxial growth having lower concentration than the dopant concentration of the wafer is conducted such as an epitaxial growth of P type film (specific resistance is 1 Ωcm) against the wafer with the dopant concentration P ++ type (specific resistance is 5 mΩcm), the dopant concentration in the epitaxial layer tends to increase at the outer peripheral portion of the wafer, as demonstrated in FIG. 4 which shows a change in dopant concentration from the center of the wafer and as a function of a distance from the center to the most outer periphery.
The above phenomenon might be due to the fact that the dopant species of the wafer 8 might be exhausted in Si—H—Cl atmosphere at the rear surface of wafer 8, and the exhausted dopant species might migrate to the front surface through the gaseous diffusion flow 11, resulting in increasing the dopant gaseous concentration locally. As a result, a particular region of the epitaxial layer where the dopant concentration is out of the range defined by the specification, leading to a poor production efficiency of the device.